Very challenging, mostly for those who are not math wiz. Anyway, it needs some instructions (say, "click on 'A', if you want to see 'down' questions"), and possibly to 'dumb down' a bit the questions...
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Solve the mathematical crosswords puzzle. Click on a tile to activate it and then insert the number (0 to 9) by pressing the corresponding keyboard button. The rules for solving the puzzle are in the top right part of the screen. Switch between across and down by clicking the black arrow. +/- to enable/disable sound. ESC to exit to the main screen.